1. Field of the Invention
The present invention relates to a magnetic device and, more particularly, to an improved electromagnetic chuck. The present invention discloses a high speed magnetizing/demagnetizing circuit used for the electromagnetic chuck.
2. Description of Related Art
In conventional machines such as electrical discharge machining (EDM) machines and grinding machines, a magnetic chuck is frequently used as a worktable, whereby a workpiece to be machined can be attracted upon the worktable by a magnetic force and then a machining operation is performed on the workpiece.
As known in the prior art, there are presently two types of magnetic chucks: permanent-magnetic chucks and electromagnetic chucks.
A permanent-magnetic chuck generally utilizes a plurality of permanent magnets. The magnetism of the permanent-magnetic chuck is determined according to the distance between the plurality of permanent magnets and the surface of the chuck as well as the arrangement of the plurality of permanent magnets.
The permanent-magnetic chuck utilizes magnets with a lower magnetic saturation point, so as to avoid resulting in significant residual magnetism on the workpiece and the surface of the chuck and thus making it difficult to remove the workpiece after the chuck is magnetically insulated. Therefore, permanent-magnetic chucks can merely be applied to machining applications with a low magnetic force demand.
An electromagnetic chuck applies the electromagnetic induction theory to generate magnetic force in order to attract a workpiece to be machined onto the surface of the chuck.
Conventionally, one drawback of the electromagnetic chuck is that it needs to be continuously provided with electric power so as to maintain its magnetic force. The magnetic units disposed within the electromagnetic chuck for generating the magnetic force gradually increase in temperature after an extended period of use. Such a rise in temperature will result in the deformation of the surface of the is electromagnetic chuck, that is, the flatness of the surface will become degraded.
In addition, the magnetic force generated by the electromagnetic chuck will dissipate when the coils of the magnetic units in the electromagnetic chuck are damaged, the power supply of the electromagnetic chuck is defective or the power is abruptly cut off, resulting in the workpiece to be machined being outwardly projected since the workpiece is still subject to the inertial force of the machining tool. Thus, it may damage the workpiece and injure the operator.
Furthermore, as the electromagnetic chuck needs to be firstly magnetized to attract a workpiece and then demagnetized after a workpiece has been machined, a magnetizing/demagnetizing circuit is usually employed. In conventional magnetizing/demagnetizing circuits, a direct current is generated to flow through the coils winding around a material to be magnetized in order to magnetize the material, and a gradually decreased voltage alternating between positive and negative via a relay is generated to apply to the coils winding around the magnetized material in order to demagnetize the material.
However, it is well known that the relay itself has several drawbacks such as operation with a time delay, large operational error and slow operational speed. The frequency of the alternating polarity of the applied voltage is low due to the slow operational speed of the relay, thus the time required to demagnetize the material is relatively prolonged (about 8 to 15 seconds).
Conventionally, the residual magnetism remaining within the magnetized material can not be minimized since the frequency of the alternating polarity provided by the relay is low. Thus, the conventional demagnetization performance is undesirable because the material may be incompletely demagnetized and the workpiece attracted onto the electromagnetic chuck may need to be separately demagnetized.